1. Field of the Invention
This invention relates to a focus detecting device.
2. Description of the Prior Art
A focus detecting device for a single lens reflex camera in which, from a light passed through the focus plate of the single lens reflex camera, a first image and a second image are formed by a pair of re-imaging lenses and the in-focus condition of an objective lens is detected from the variations in position of said first and second images with respect to a pair of image position detecting photoelectric converters disposed on or near the focal planes of said pair of re-imaging lenses is already known, for example, from Japanese Laid-open Patent Application No. 7323/1979.
However, the prior art has the following disadvantages which will hereinafter be described by reference to the accompanying drawings. FIG. 1 is an illustration of the focus detecting device according to the prior art. In FIG. 1, the light beam passed through a phototaking objective lens 1 is converged on the focal plane 2 of a focus plate and then diverged, and part of the light beam passed through the objective lens is converged and imaged on the light-receiving surfaces of a pair of photoelectric elements 4 and 4' by a pair of re-imaging lenses 3 and 3' disposed symmetrically with respect to the optic axis of the objective lens 1. The pair of photoelectric elements 4 and 4' act as image position detecting photoelectric converters and specifically comprise a photoelectric element array. It should be noted here that the distributions of the intensities of illumination on the light-receiving surfaces of the photoelectric elements 4 and 4' for an object must be equal to each other. This is because the focus detection is accomplished by comparing the photoelectric outputs for the same region of an image (light image).
Now, when light beams impinging on the upper light-receiving surface 4a, the central light-receiving surface 4c and the lower light-receiving surface 4b of the photoelectric element 4 are conversely projected upon the exit pupil of the object lens 1 from these light-receiving surfaces through the re-imaging lens 3, they become such as shown in FIG. 1. That is, a light beam forming an opening a.sub.1 a.sub.2 a.sub.3 mpinges on the upper light-receiving surface 4a, and a light beam forming an opening c.sub.1 c.sub.2 c.sub.3 impinges on the central light-receiving surface 4c. The openings a.sub.1 a.sub.2 a3 and c.sub.1 c.sub.2 c.sub.3 are determined by the effective F-number of the re-imaging lens 3 and are substantially equal in size. However, when converse projection is effected upon the lower light-receiving surface 4b in the same manner as upon the former two light-receiving surfaces, the light beam which should impinge on the light-receiving surface 4b through the re-imaging lens 3 has an opening b.sub. 1 b.sub.2 b.sub.3, but when the pupil diameter of the objective lens is small, part of the light beam is missed at the upper end of the objective lens 1 and only a light beam forming an opening b.sub.1 ' b.sub.2 b.sub.3 can impinge on the lower light-receiving surface 4b. This means that, for example, when an object having a uniform distribution of brightness is measured, the intensity of illumination on the lower light-receiving surface 4b is reduced as compared with the intensities of illumination on the upper light-receiving surface 4a and the central light-receiving surface 4c. That is, a distribution of intensity of illumination corresponding to the brightness distribution of the object cannot be obtained on the light-receiving surface of the photoelectric element 4.
This phenomenon equally occurs to the other photoelectric element 4' and the intensity of illumination on the upper light-receiving surface 4'a on which the light beam missed at the lower end of the objective lens 1 impinges is reduced below the intensities of illumination on the central light-receiving surface 4'c and the lower light-receiving surface 4'b. Now, the light image formed on the light-receiving surface 4b and the light image formed on the light-receiving surface 4'a are identical, but the reproducibility of the intensity of illumination of the light image for the object differs from region to region and therefore, the distributions of brightness of the images on the photoelectric elements 4 and 4' do not become identical. Accordingly, the photoelectric outputs of the photoelectric elements 4 and 4' cannot be properly compared and thus, there occurs a focus detection error.
As a technique which overcomes this disadvantage, Japanese Laid-open Patent Application No. 95624/1978 (corresponding German Patent Application P2703290) discloses a technique whereby a field lens is provided near the focus position of the phototaking objective lens. According to this technique, the aforementioned light beams shown in FIG. 1 are displaced toward the optic axis by the field lens. Describing this by reference to FIG. 2, all the light beams b.sub.1 -b.sub.3 converged on the light-receiving surface 4b of the photoelectric converter 4 pass through the pupil of the objective lens 1 due to the presence of the field lens 6a provided on the focus plate 6. Of course, the light beams converged on the light-receiving surface 4'a of the photoelectric converter 4' also pass through the pupil of the objective lens 1. Also, of the light beams which have already passed through the pupil of the objective lens, the light beam converged on the light-receiving surface 4a or 4'b is displaced toward the optic axis (for example, like a.sub.1 -a.sub.3). The light beam c.sub.1 -c.sub.3 passing through the center of the field lens 6a remains unchanged. In this manner, the vignetting at the objective lens 1 can be eliminated, but there is left the following disadvantage.
That is, the power of the field lens must be determined in accordance with the optical characteristic of the objective lens to which the field lens is directed. Accordingly, the curvature of the field lens is determined by the objective lens. On the other hand, the size of the field lens is determined substantially correspondingly to the area, on the focal plane, of an object whose range is to be found, and therefore said curvature cannot be secured in some cases. In FIG. 3, for example, when the field lens 6a is shaped integrally with the focus plate 6 and if the diameter of the field lens (namely, the size of the object whose range is to be found) is d.sub.1 with the assumption that the curvature of the field range is determined to a certain value, it will be possible to manufacture the field lens, but if the diameter of the field lens is d.sub.2, it will be impossible to secure the edge thickness the field lens and to manufacture the field lens. According, in this case, the curvature of the field lens must be made great and the correction as shown in FIG. 2 will become incomplete. That is, as regards a certain type of objective lens, the use of it will still cause a focus detection error to occur.